Die and device for positioning a joining element or for clinch joining

ABSTRACT

A die for a device for joining is proposed, wherein at least one monitoring element for monitoring the functionality of the die is attached to the die, wherein the monitoring element comprises a strain gauge, wherein the monitoring element comprises exactly two sensors, wherein the spring means have a cross spring with multiple elastically movable spring arms, wherein one die portion is assigned to one spring arm, and wherein the spring arms are connected at one point, wherein the die has four movable die portions which are elastically movable in each case by means of one spring arm of the spring means.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of International Application No. PCT/EP2017/070965 filed Aug. 18, 2017, which designated the United States, and claims the benefit under 35 USC § 119(a)-(d) of German Application No. 10 2016 115 595.0 filed Aug. 23, 2016, the entireties of which are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a die and device for positioning a joining element or for a clinch joining.

BACKGROUND OF THE INVENTION

Devices for joining or for placing a joining element on a workpiece or for clinch joining of workpieces without an additional element are known in a variety of embodiments. The respective devices generally comprise a punch unit with a driveable punch and a die, wherein the workpiece can, for the working, be positioned between the punch unit and the die.

By means of the stated devices, it is possible for functional elements or rivets, solid punch rivets, semi-hollow punch rivets and clinch rivets to be applied to the workpiece.

Depending on the joining task or on the workpiece to be worked or on the additional element, different demands must be satisfied by the device and in particular by the die. In the case of dies with die portions which move during the joining, the dies are structurally more cumbersome, and the problem areas are more complex.

It has hitherto been the case that modern or practical solutions have not been available for all problem situations in the present applications.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a device and a die of the type mentioned in the introduction which constitute a practical improvement in relation to known arrangements, in particular, with regard to modern environments with high demands on continuous process control in applications with high cycle rates or fully automated working processes.

The present invention is based on a die for a device for joining, for example, for punch riveting, clinching or clinch riveting. According to a first aspect of the present invention, at least one monitoring element for monitoring the functionality of the die is attached to the die. It is thus possible to directly monitor the component, specifically the die, at which states and processes which are critical for the overall process are primarily to be expected. The monitoring element is designed in a manner exactly adapted for this as regards specifically the function and/or the construction of the die.

The monitoring element preferably operates independently of the punch unit.

The function of the die necessitates the action of a joining force during the joining process on the workpiece or on the rivet. A build-up of force during the joining process occurs between punch and die. The punch is movable relative to the die along a joining axis of the die or of the joining device, wherein the workpiece is positioned on the die and/or is supported on the die.

The monitoring element is preferably designed to continuously detect the state or the function of the die and provide or transmit corresponding items of information. The monitoring element is preferably a separate component with respect to the die, or the monitoring element is alternatively a component provided integrally on the die.

The monitoring element is advantageously attachable or provided separately from or independently of die portions which are subject to wear. The monitoring element itself is thus usable, or protected against adverse effects, over a long service life.

In particular, with the present invention, it is possible in a simple manner to not only distinguish between die states of “functional” and “non-functional” but also identify states prior to failure or prior to non-functionality of the die.

The provision of the information relating to the state of the die from the monitoring element to the surroundings may be realized in various ways, for example, by transmission to a superordinate unit such as a monitoring or closed-loop or open-loop control unit and/or through the provision of a warning or state signal to the surroundings or to the overall system.

The monitoring element is, for example, designed such that detectable states and/or functions are provided by means of a binary item of information, for example, in the manner of “OK” or “not OK”.

Information provision by means of the monitoring element is, however, also possible which provides in each case exactly one item of information content selected from a multiplicity of mutually different items of information content. For example, various items of information content may be provided which distinguish between states, such as for example “damaged”, “worn”, “jammed” and the like, whereby a statement is made in each case with regard to a particular part and/or a particular function of the die. One item of information may, for example, relate to the state and/or the function of a movable die portion. Another item of information permits, for example, an assessment of the state and/or of the function of a positionally fixed die portion. A yet further item of information makes it possible, for example, for conclusions to be drawn regarding the state and/or the function of spring means of the die.

The monitoring of the die and also the provision or transmission of the information to the superordinate unit or to the surroundings may be continuous, or may be performed at intervals.

The monitoring unit is, for example, a computer-based monitoring unit which preferably monitors that device for placing the joining element or for joining at which the die is provided.

According to the present invention, the monitoring element comprises a strain gauge. With a strain gauge, a mechanically loaded element of the die can be continuously and precisely monitored in a space-saving manner. The flat form and the small thickness of the strain gauge makes it possible for the strain gauge to be accommodated on the die without problems. Furthermore, in particular, contacts for contacting with counterpart contacts of a line arrangement are provided on the strain gauge.

With at least one strain gauge, wherein it is preferable for multiple strain gauges to be provided, it is possible, in particular, for stresses in the secondary structure or damage in the die portion coupled to the strain gauge to be detected, and for deviations from a standard characteristic to be identified. Preferably, the strain gauge is coupled to spring means which act on, or are, in particular, in contact with, the movable die portions. If damage to or a malfunction of the die or a movable die portion and/or the spring means themselves occurs, this has an effect on the stress characteristics of the spring means detected by the strain gauge.

In particular, a damaged movable die portion has the effect that part of the spring means which is assigned to the damaged movable die portion behaves differently from the non-damaged standard situation. This leads to a non-standard stress state of the spring means or to an uneven stress distribution in the spring means, which lies outside a tolerable range. The uneven stress distribution in the spring means is identified or detected by the preferably multiple strain gauges, which are preferably attached to the spring means, wherein a corresponding detection signal may be transmitted from the strain gauge to a superordinate unit.

According to the present invention, the monitoring element comprises exactly two sensors. It is thus possible to realize reliable functional monitoring with relatively little outlay. It is preferable for exactly two sensors or strain gauges or exactly three or exactly four sensors or strain gauges to be provided, in particular, if the die has two or three or four movable die portions with in each case one associated spring portion, acting thereon, of the spring means.

The spring means have multiple elastically movable spring arms, wherein one movable die portion is assigned to one spring arm. It is preferable for multiple movable die portions to be provided on the die, wherein each die portion is acted on by in each case exactly one identical portion of the spring means, for example, one spring arm of a multi-arm yoke spring.

The die has four movable die portions which are elastically movable in each case by means of one spring arm of the spring means. It is thus possible for a die for a symmetrical joining point to be provided, in particular, with four die portions which are deflectable radially outward with respect to an axis of the die and with four spring arms which correspondingly deflect radially. The spring arms are preferably formed, at one of four bent and connected strip-like spring arms, as a cross spring with four spring arms connected at one point. The die is preferably of symmetrical construction. According to a further aspect of the present invention, at least one monitoring element for monitoring the joining process that it is intended to be able to carry out by means of the die is attached to the die. It is thus possible, irrespective of the state of the die, or in the case of non-damaged or worn dies, to identify states which are attributable to improper use of the die. This may, for example, be the case, proceeding from a new or ideal die, if the workpiece or various workpiece layers are positioned obliquely or in a deviating manner in relation to a correct orientation or correct position on the die. This may, for example, lead to an uneven movement of the movable die portions relative to one another and/or to an uneven bending-up of the elements of the spring means, which can be identified, and assigned to the corresponding error, for example, by way of a comparison of actual and setpoint characteristics, by the monitoring element.

Furthermore, by means of the monitoring of the joining process, it is advantageously possible for conclusions to be drawn regarding the nature or quality of the deformation performed on the workpiece, such as a joining point. For example, in the case of clinching or clinch riveting, conclusions can be drawn regarding an undercut of workpiece layers that has been achieved at the respective joining point.

It is furthermore advantageous if the monitoring element is coupled to a die portion of the die which, during the joining, is subjected to a movement caused by the joining process, wherein the monitoring element monitors, in particular, the movement of the movable die portion. By means of the coupling of the monitoring element to the movable die portion, it is possible for the state of a movable die portion to be directly detected, and for an associated item of information to be provided reliably and with high informative value. This is advantageous inter alia because a movable die portion generally forms the critical part of a die. This is because a movable die portion is dynamically and mechanically relatively highly loaded. Movable die portions which function faultlessly are a prerequisite for the functioning of the die. Therefore, problems during the joining are commonly attributable to incorrectly functioning or damaged or entirely absent movable die portions, whereby the problem identification is performed in an advantageous manner with the die according to the present invention.

A coupling of the monitoring element to the die portion of the die is to be understood to mean various relationships which are each adapted to the respective die in a manner dependent on the individual situation. A coupling may, for example, mean a direct, for example, physical connection or an indirect connection between the monitoring element and the die portion. The coupling may furthermore be permanent or non-permanent. It is also conceivable for the nature of the coupling to change during the course of the joining process, for example, for an indirect coupling between the monitoring element and the movable die portion to be formed in a first operating phase of the die, and for a direct coupling, for example, with abutting contact, between the monitoring element and the die portion to be formed at a later point in time.

A coupling is, for example, abutting contact between the monitoring element and the movable die portion, wherein the monitoring element is configured such that the abutting contact occurs when the movable die portion passes through a predefinable movement travel, which is particularly informative with regard to a malfunction or a possible damaged state of the die. The movement travel of the die portion that is monitored by means of the coupling is preferably a final part of the travel assumed by the movable die portion during a joining process before an end position or standstill position of the movable die portion is reached. Damage to the die can commonly be identified from the fact that the movement travel of the movable die portion deviates from a standard travel that has been covered. The associated deviation is identified from the die according to the present invention by means of the monitoring element.

It is also advantageous that the monitoring element is coupled to a die portion of the die which is provided movably on the die.

In the event of damage, the movable die portion, for example, over its final portion of the movement travel, may not reach the correct or standard end position or may not come to a standstill in the end position, but may rather come to a standstill before or after the end position, or may perform a non-standard or incorrect movement, for example, faster or slower or not uniform but jerky, compared with the functionally correct standard situation.

For example, in the event of damage to the die, a movable die portion, or multiple movable die portions, may move beyond an end position which, in the damage-free situation, is a correct end position of the movement travel of the die portion.

The movable die portion may be a movable segment of the die which interacts with the workpiece, and/or a part of spring means which acts on a movable segment of the die, for example, a yoke spring, an elastomer ring or some other mechanical spring.

It is furthermore advantageous that the monitoring element is coupled to a secondary structure, wherein the secondary structure interacts with a die portion of the die which is provided movably on the die. The secondary structure is, for example, formed by spring means for providing a force or a moment on the movable die portion. For example, the secondary structure comprises portions which are themselves provided movably on the die. The secondary structure is preferably relatively easily accessible on the die, or forms, for example, a region of an outer side of the die or is attached to an outer side of die portions.

In one advantageous embodiment of the present invention, the monitoring element comprises a sensor arrangement. By means of a sensor arrangement, for example, with commercially available sensors, the die can be monitored in an economically advantageous manner. In particular, various parameters of interest for the functional monitoring can be reliably detected and fed to a superordinate unit, for example in wired fashion.

Alternatively or in addition, the monitoring element advantageously has a force sensor such as, for example, a piezo element.

It is moreover advantageous if the monitoring element comprises a sensor such as a strain gauge which serves for continuously providing measurement signals for the monitoring of the functionality of the die and/or of the joining process that can be carried out by means of the die. Thus, reliable die and process monitoring is possible throughout the entire joining process. A malfunction of the die or in the joining process is identified immediately. More extensive technical damage or economic losses can thus be ruled out.

A further advantage of the present invention arises if the monitoring element is configured to, in the event of the die portion that is coupled to the monitoring element being subjected to a load that lies outside a normal load state, provide an associated measurement signal. Thus, at least in critical states of the die, which generally means a critical state of the joining process, a measurement signal is generated. A malfunction or a failure of the die is thus identified. It is preferable that further actions or measures may possibly be automatically initiated by means of the measurement signal, whereby high process reliability is possible.

It is also advantageous for the monitoring element to be designed to provide a measurement signal to a superordinate unit. The superordinate unit is preferably a monitoring unit, for example, for process control. The measurement signal is provided, in particular, as an electrical signal. By means of the superordinate unit, it is possible for predefined follow-up measures to be initiated quickly and effectively.

In a further advantageous variant, the monitoring element comprises multiple sensors. It is preferable for multiple individual sensors of the same type, or multiple identical sensors, to be provided. This is advantageous, in particular, in the case of dies of symmetrical construction. This is because it is then possible for deviations in symmetry in the movement profile to be identified. For example, multiple symmetrically arranged parts of the movable die portions move identically during the joining process in the standard situation. Damage to or a malfunction of one part of the multiple parts of the movable die portions is advantageously detectable by the sensor as a deviation from the standard situation or standard characteristics assigned to the respective part to which the sensor is, for example, attached. The respective part behaves differently than in the standard situation or differently than the other or non-damaged parts of the movable die portions.

This applies correspondingly to fixed die portions, wherein in the event of damage to a fixed die portion, this is identified.

For example, multiple strain gauges are provided on the die, and, in particular, in each case one associated sensor is provided for each movable die portion. In addition or alternatively, a respectively associated sensor may be provided for preferably each spring element of spring means composed of multiple spring elements.

As a sensor, use may be made, for example, of a strain gauge and/or of a piezo element.

It is also advantageous if the monitoring element comprises exactly three or exactly four sensors. It is thus also possible for relatively complex dies with multiple fixed and/or multiple movable die portions to be monitored with high informative value.

The die advantageously has a movable die portion which interacts with spring means for a resettable movement. Accurately repeatable movement and mounting of the movable die portion, for example, of a die segment, can thus be predefined.

According to another advantageous modification of the invention, the die has spring means with an elastically flexible spring arm. A spring arm can be accommodated in a compact manner, or can, through the specification of the dimensions and of the material, be provided with different stages of action.

The present invention furthermore extends to a device for placing a joining element on a workpiece or for clinch joining of the workpiece, comprising a punch unit and an oppositely situated die unit, between which the workpiece that can be worked by means of the device can be clamped, wherein the die unit comprises a die according to one of the embodiments described above. The device according to the present invention is, for example, a clinching or riveting tool with a C-shaped or O-shaped bracket, or a column frame, on which a punch unit and an oppositely situated die are provided. The device can be used preferably for the processing of rivets such as, for example, clinch rivets and/or for clinching or clinch joining.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the present invention will be discussed in more detail on the basis of the arrangement according to the present invention illustrated in the figures, in which, in detail:

FIG. 1 shows a highly schematic detail of a known clinching device with a two-layer workpiece at the start of a clinching process, wherein parts of the clinching device have been omitted;

FIG. 2 shows the arrangement as per FIG. 1 at the end of the clinching process;

FIG. 3 shows the machined workpiece as per FIG. 2 after the clinching process;

FIG. 4 shows a die according to the present invention in a side view;

FIG. 5 shows the die as per FIG. 4 in longitudinal section;

FIG. 6 shows the die as per FIG. 4 from above;

FIG. 7 shows an insert of the die as per FIG. 4 in a perspective view obliquely from below; and

FIG. 8 shows, on its own, spring means of the die as per FIG. 4 with a monitoring element.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 highly schematically shows a known device 1 for the clinching or clinch joining of a workpiece 4 immediately before a deformation process of the workpiece 4. The clinching device 1 has a punch unit 2 and an oppositely situated die 3, between which the workpiece 4 is positioned or placed on the die 3. The workpiece 4 comprises an upper workpiece layer 5, at the side of the punch, and a lower workpiece layer 6, at the side of the die. The workpiece layer 5 and the workpiece layer 6 are, for example, in each case a plastically deformable metal sheet. By means of the clinching device 1, the two workpiece layers 5 and 6 are deformed at a connecting point 13, such that, after the deformation, the two workpiece layers 5 and 6 are fixedly connected to one another. Here, an undercut HS of material regions of the upper workpiece layer 5 relative to the lower workpiece layer 6 is formed.

The punch unit 2, at its front free end directed toward the die 3, comprises a punch mandrel 7. The punch mandrel 7, which in FIG. 1 is positioned with its face side on a top side of the workpiece layer 5 which has not yet been deformed, can be acted on with force or is movable in driven fashion along a joining axis S in a direction P1 toward the die 3 and, after a clinching process, back in the direction P2, for which purpose a drive unit (not shown) is provided, for example, an electric or hydropneumatic drive unit.

The die 3 comprises an inner, positionally fixed, cylindrical inner part 8, on which, at the top side, there are supported four movable elements which are offset at 90 degree angles circumferentially about the joining axis S and of which two opposite elements 9 and 10 are visible.

The elements 9 and 10 are held so as to be movable radially outward, as per P3, and radially inward, as per P4, with respect to the joining axis S. For this purpose, spring means are provided, which press in preloaded fashion against the movable elements 9 and 10 at the outside. The spring means are formed by elastically flexible spring arms 11 and 12. The spring arms 11 and 12 are, in the initial position (FIG. 1), toward their free end or in the direction P2, oriented slightly obliquely inward toward the joining axis S.

FIG. 2 shows the operating state at the end of the clinching process when the punch mandrel 7 has been moved to a maximum extent in the direction P1, and the elements 9 and 10 have been deflected in the direction P3, wherein the spring arms 11, 12 have been bent elastically outward in the direction P3 relative to the initial position as per FIG. 1, and extend approximately parallel to the joining axis S.

When the punch mandrel 7 moves back in the direction P2, the elements 9 and 10 are reset again into the initial position as per FIG. 1 in the direction P4 by the spring force of the spring arms 11, 12.

The movable elements 9 and 10 are provided between fixed segments (not illustrated) and enable the deformed material of the workpiece 4 to form the undercut HS, or to flow behind, at the connecting point 13.

For the formation of the connecting point 13, it is fundamental that the spring arms 11, 12 operate faultlessly, and in particular exhibit no damage. The device 1 operates functionally correctly, in particular, if all of the spring arms 11, 12 provided on the die, or the multiple spring arms 11, 12, are adjustable in a predefined manner, and preferably each provide an identical spring force or an identical spring force profile during the clinching process.

An incorrect orientation or positioning of the workpiece on the die, and/or a joining point which is not formed optimally during the monitored joining process, are also disadvantageous, and can be reliably and practically identified by means of the present invention.

Furthermore, damage to, wear of and/or fouling of the die or the spring arms 11, 12 can adversely affect the function of the spring arms 11, 12, which leads to an incorrectly functioning die 3, which in turn results in an incorrectly formed or defective connecting point 13, which in particular does not provide the required connection stability of the connection of the workpiece layers 5 and 6.

This is addressed by the present invention, wherein, in the joining process or during the operation of the joining device, the monitoring of the function of the die is realized by means of a monitoring element.

FIGS. 4 to 8 relate to a die 14 according to the present invention for an associated clinching tool for the clinching or clinch joining of a workpiece W which has not yet been deformed, as indicated by dashed lines in FIG. 4. By means of the die 14, a clinching point which is symmetrical with respect to a joining axis S is formed on the workpiece W.

By means of the die 14, it is, in particular, possible for a punctiform elevation to be produced on the underside, at the side of the die, of the workpiece W, and flexible use for clinching or clinch joining of different thicknesses of the workpiece W with one toolset is possible. The die 14 permits a relatively space-saving, slim form of the associated clinching device, which is distinguished by high accessibility and the fewest possible obstructive contours on the die 14.

The die 14 has a die main body 15 and a die insert 16 inserted into the upper part of the die main body 15, which die insert comprises an upper part 16 a and four lamellae 21, 22, 23 and 24 held movably thereon. Between the die main body 15 and the die insert 16, there are provided spring means, which are formed, for example, as a yoke spring or as a cross spring 25. The cross spring 25, composed, for example, of spring steel, has a cross-shaped lower part 30, on which four strip-like spring arms 26, 27, 28 and 29 protrude angularly upward via a bend radius. Instead of the cross spring 25, the spring means may also be designed differently, for example, may comprise an elastomer ring, a helical spring or other technical springs. The die insert 16 is fixed in its correct position on the die main body 15 by means of a clamping pin 34 which can be inserted through a corresponding opening. By means of the fixed die insert 16, the cross spring 25 is also fixed on the die 14.

For monitoring the functionality of the die 14, a monitoring element 31 is provided on the underside on the lower part 30 of the cross spring 25. The monitoring element 31 is formed for example from two strip-like strain gauges 32 and 33 which are arranged crosswise and which, by way of their respective end portions 32 a, 33 a composed of two electrically conductive wires, are led on to the outer side of the die 14 or are coupled thereto. The strain gauges 32, 33, which are, for example, adhesively bonded to the underside of the lower part 30, extend on the underside of the lower part 30 over the major part of the length of the four portions of the lower part 30 which are oriented crosswise, wherein each of the four portions transitions into in each case one of the four spring arms 26-29. Thus, the strain gauges 32, 33 detect all stress states of the entire cross spring or of the four spring arms 26-29, in particular, if the spring arms 26-29 are deformed or perform a movement during the joining process, which normally means an elastic deflecting movement outward in the direction P3 and back in the direction P4.

An uneven stress distribution of the four spring arms 26-29 that can be detected by the strain gauges 32, 33 results, for example, in the event of damage to one or more of the lamellae 21-24 and/or to the cross spring 25 or to at least one spring arm of the spring arms 26-29. The associated measurement signals of the two strain gauges 32, 33 are transmitted via lines (not illustrated), which engage on the end portions 32 a, 33 a, to the monitoring unit, and are processed there, for example, in order to output a warning signal and/or an error message.

The die main body 15 comprises a solid cylindrical base 35 with a thin-walled or sleeve-like enclosure 36 adjoining it at the top. In the open side, facing toward a punch of the associated clinching tool, of the enclosure 36, four slots 37 which are offset with respect to one another in a circumferential direction at 90 degree angles are formed in from above, which slots extend over the major part of the length of the enclosure 36. The enclosure 36 surrounds a receiving space for accommodating the die insert 16 and the cross spring 25. The assembly or the disassembly of the die 14 composed of the parts 15, 16, 25, 32, 33 and 34 is possible in a small number of easy steps.

The construction of the top side of the die 14 can be clearly seen in particular from FIGS. 5 and 6. Between the four lamellae 21-24, which are U-shaped in the section as per FIG. 5, in a circumferential direction, there are provided four fixed portions of the upper part 16 a, which during a clinching process each provide a planar support region 17, 18, 19 and 20 for the workpiece W that can be machined using the associated clinching tool. The respective planar top sides of the lamellae 21-24 are provided at the same level as, or in a plane E with, the support regions 17-20. The workpiece W, which is generally planar on the bottom side, is supported at the start of the clinching process at the underside on the top of the support regions 17-20 and on the top sides of the lamellae 21-24, which lie in the plane E (see FIG. 5). An upwardly open die depression 38 of the die 14 is surrounded by the support regions 17-20 and the lamellae 21-24 and forms a central depression with a depth which predefines a projecting length, at the underside, of a clinched connecting point, producible by means of the die 14, on the respective workpiece W.

In the case of a functionally correctly operating die 14, in order to produce a symmetrical clinching or connecting point, the lamellae 21-24 and thus the spring arms 26-29 deflect radially, or in the direction P3, as far as a predefinable deflection position. This is realized during the clinching process by means of a punch action of the punch (not shown), which is movable in driven fashion along the joining axis S, of the associated clinching tool, wherein deformed workpiece material forces the lamellae 21-24 radially outward, for example, to such an extent that all four spring arms 26-29, or the longitudinal axes thereof, are parallel to the joining axis S, or are, for example, in alignment with the wall of the enclosure 36. Here, the spring arms 26-29 plunge from radially inside outward into the material-free region of the slots 37, which accordingly provide in each case a deflection movement travel of the spring arms 26-29.

With the strain gauges 32, 33 attached to the cross spring 25, damage to or a malfunction of the die 14 is immediately and directly identified in particular in the process environment of the clinching device. Thus, according to the present invention, functional monitoring, or an indication of a malfunction of or damage to the die 14, is reliably provided.

LIST OF REFERENCE DESIGNATIONS

-   1 Device -   2 Punch unit -   3 Die -   4 Workpiece -   5, 6 Workpiece layer -   7 Punch mandrel -   8 Inner part -   9, 10 Element -   11, 12 Spring arm -   13 Connection point -   14 Die -   15 Die main body -   16 Die insert -   16 a Upper part -   17-20 Support region -   21-24 Lamellae -   25 Cross spring -   26-29 Spring arm -   30 Lower part -   31 Monitoring element -   32 Strain gauge -   32 a End portion -   33 Strain gauge -   33 a End portion -   34 Clamping pin -   35 Base -   36 Enclosure -   37 Slot -   38 Die depression 

1. A die for a device for joining, wherein at least one monitoring element for monitoring the functionality of the die is attached to the die, wherein the monitoring element comprises a strain gauge, wherein the spring means have a cross spring with multiple elastically movable spring arms, wherein one die portion is assigned to one spring arm, and wherein the spring arms are connected at one point, wherein the die has four movable die portions which are elastically movable in each case by means of one spring arm of the spring means.
 2. The die as claimed in claim 1, wherein at least one monitoring element for monitoring the joining process that it is intended to be able to carry out by means of the die is attached to the die.
 3. The die as claimed in claim 1, wherein the monitoring element is coupled to a die portion of the die which, during the joining, is subjected to a movement caused by the joining process, wherein the monitoring element monitors the movement of the movable die portion.
 4. The die as claimed in claim 1, wherein the monitoring element is coupled to a die portion of the die which is provided movably on the die.
 5. The die as claimed in claim 1, wherein the monitoring element is coupled to a secondary structure, wherein the secondary structure interacts with a die portion of the die which is provided movably on the die.
 6. The die as claimed in claim 1, wherein the monitoring element comprises a sensor arrangement.
 7. The die as claimed in claim 1, wherein the monitoring element comprises a sensor such as a strain gauge which serves for continuously providing measurement signals for the monitoring of the functionality of the die and/or of the joining process that can be carried out by means of the die.
 8. The die as claimed in claim 1, wherein the monitoring element is configured to, in the event of the die portion that is coupled to the monitoring element being subjected to a load that lies outside a normal load state, provide an associated measurement signal.
 9. The die as claimed in claim 1, wherein the monitoring element is designed to provide a measurement signal to a superordinate unit.
 10. The die as claimed in claim 1, wherein the monitoring element comprises multiple sensors.
 11. The die as claimed in claim 1, wherein the monitoring element comprises exactly two sensors.
 12. The die as claimed in claim 1, wherein the monitoring element comprises exactly three or exactly four sensors.
 13. The die as claimed in claim 1, further comprises a movable die portion which interacts with spring means for a resettable movement.
 14. The die as claimed in claim 1, further comprises spring means with an elastically flexible spring arm.
 15. A device for placing a joining element on a workpiece or for clinch joining of the workpiece, comprising a punch unit and an oppositely situated die unit, between which the workpiece that can be worked by means of the device can be clamped, wherein the die unit comprises a die as claimed in claim
 1. 16. A die for a device for joining wherein at least one monitoring element for monitoring the joining process that it is intended to be able to carry out by means of the die is attached to the die.
 17. A device for placing a joining element on a workpiece or for clinch joining of the workpiece, comprising a punch unit and an oppositely situated die unit, between which the workpiece that can be worked by means of the device can be clamped, wherein the die unit comprises a die as claimed in claim
 16. 